Method and apparatus for monitoring film thicknesses by sensing magnetic interaction between members movable to a film thickness distance

ABSTRACT

An improved method and apparatus for monitoring the thickness of a film includes a sensor roll which cooperates with a compliant roll to form a nip through which the film passes. As the two rolls rotate relative to each other, magnets disposed in the sensor roll are sequentially moved to and from the nip. As a magnet moves by the nip, the field emanating from the magnet is concentrated by a body of material having a relatively high magnetic permeability and disposed in the compliant roll. This results in an induced voltage being generated in a sensor coil. The output from the sensor coil is transmitted to control circuitry which effects operation of a suitable apparatus to control the thickness of the film.

This is a continuation of co-pending application Ser. No. 560,324 filedon Dec. 12, 1983 now U.S. Pat. No. 4,661,774.

BACKGROUND OF THE INVENTION

The invention relates to a method and an apparatus for monitoring thethickness of a film. More specifically, the invention relates to amethod and an apparatus for providing information representative of thethickness of an ink film in a printing press.

Many known devices provide information about the thickness of a film. Insome known devices, electrical transducers are used. The transducers maybe of the type which have electrical properties that vary in dependenceupon the thickness of the film. The transducer may sense changes ininductance, capacitance or resistance caused by changes in the thicknessof the film passing thereby.

U.S. Pat. No. 3,857,095 discloses an electromagnetic sensor whoseinductance changes with changes in the thickness of an ink film on aroller which supplies ink to a printing roll. In U.S. Pat. No.4,345,203, a capacitance transducer is used for measuring the thicknessof a film of lubricant between two relatively rotatable surfaces. Withthese known devices, the measured electrical properties may varysignificantly with variations in the electrical properties of the film.Therefore, the measured properties could vary significantly withvariations in density, water content, temperature, etc. of a liquidfilm.

One of the ways to alleviate these problems is to use magneticreluctance sensing. Under this method, a magnetic flux is establishedbetween a member that carries the film and a member that carries amagnetic transducer for sensing variations in the film thickness. Such atransducer generally includes a core and an excitation coil associatedwith the core to establish a magnetic flux across the film. A sensingmeans senses variations in the magnetic flux resulting from changes inthe thickness of the film. Such devices are shown for example in U.S.Pat. No. 3,922,599.

A magnetic transducer for measuring the thickness of a lubricant film ina bearing is described in the French article "Measuring without AContact", CETIM-Informations, No. 76. In this transducer, a U-shapedcore is employed with core legs facing the film of lubricant. Excitationand sensing coils are mounted around the core.

It is known that the reluctance of a magnetic circuit is: ##EQU1##where: A is the area of the pole face,

M_(o) is the free space permeability,

M_(e) is the permeability of the magnet alloy,

L_(m) is length of the magnetic core, and

d is the distance or gap from the core to the liquid film carryingmember. (The gap appears at each pole of a U-shaped core, hence thefactor of 2.)

Generally, M_(e) is significantly greater than L_(m), therefore, thefirst term in brackets is quite small and can be neglected.

The magnetic flux is determined from an equation: ##EQU2## where F isthe magneto-motive force and is equal to N_(e) I, where N_(e) is numberof turns of the excitation coil, and I is the current. Substituting Rfor its expression, the magnetic flux will be: ##EQU3## The voltageinduced in the sensing coil is: ##EQU4## where N_(S) is number of turnsin the sensing coil. Substituting dQ for its expression (Equation 3) theinduced voltage is: ##EQU5## In known methods, d is generally keptconstant, and generation of voltage occurs by changing the current, I.

Many prior art film thickness measuring devices use alternating currentto energize an excitation coil of an electromagnet. However, the use ofalternating current results in parasitic flux in the transducer. Thepresence of the parasitic flux in a transducer does not allow a numberof transducers to be connected in series. Such a series connection isgenerally advantageous when the thickness of a liquid film has to bedetermined at predetermined positions across the width of the film.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a method and apparatus for monitoring thethickness of a film. The apparatus includes a sensor roll and acompliant roll which cooperate to define a nip through which the filmpasses. Each magnet of a series of magnets on the sensor roll is movedin turn past the nip. Each of the magnets is of a constant magneticexcitation and cooperates with a body of material having a relativelyhigh magnetic permeability as it passes by the nip. As the magnet ismoved by the nip, the magnetic field is concentrated and an inducedmagnetic flux change is generated in a sensor coil. The maximummagnitude of the induced magnetic flux change varies as an inversefunction of the thickness of the film at the portion of the nip betweenthe magnet and the compliant roll.

Although a film thickness monitor assembly constructed in accordancewith the present invention can be used in many different environments todetect the thickness of either liquid or solid films, the assembly isadvantageously used in a printing press to detect the thickness of aliquid film of ink applied to a plate cylinder. When the film thicknessmonitor assembly is used in a printing press, the output from the sensorcoil is transmitted to control circuitry which effects operation of inkfountain keys to maintain a desired film thickness. The magnets areadvantageously arranged in an array in the sensor roll to obtain aseries multiplexing action due to movement of each magnet in apredetermined pattern past the nip. A preferred embodiment would be ahelical array although any other providing a decodable input wouldsuffice.

Accordingly, it is an object of this invention to provide a new andimproved method and apparatus for monitoring the thickness of a film andwherein a magnet is moved relative to a body of material having arelatively high magnetic permeability to generate an induced voltagewhich is a function of the thickness of the film.

Another object of this invention is to provide a new and improved methodand apparatus for monitoring the thickness of a film which moves througha nip formed between a pair of rolls and wherein an induced voltagehaving a magnitude representative of the thickness of the film isgenerated by rotating the magnet with one of the rolls.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the present inventionwill become more apparent upon a consideration of the followingdescription taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic illustration of a printing press inker having afilm thickness monitor assembly constructed in accordance with thepresent invention;

FIG. 2 is a partially broken away schematic fragmentary illustrationdepicting the relationship between a sensor roll and a compliant roll inthe film thickness monitor assembly used in the inker of FIG. 1;

FIG. 3 is a sectional view, taken generally along the line 3--3 of FIG.2, illustrating the relationship between a magnet in the sensor roll anda ring of material having a relatively high magnetic permeabilitydisposed on the compliant roll, the magnet being disposed adjacent a nipbetween the sensor and compliant rolls;

FIG. 4 is an enlarged fragmentary schematic illustration depicting therelationship between an ink film, a magnet disposed on the sensor rolland a ring of material having a high magnetic permeability on thecompliant roll as the magnet moves past the nip between the sensor andcompliant rolls;

FIG. 5 is a schematic illustration of control circuitry used in the filmthickness monitor assembly;

FIG. 6 is a graph depicting the relationship between an induced magneticflux change generated in a sensor coil and movement of a magnet past thenip between the sensor and compliant rolls; and

FIG. 7 is a fragmentary illustration of a second embodiment of thecompliant roll.

DESCRIPTION OF SPECIFIC PREFERRED EMBODIMENTS OF THE INVENTION GeneralDescription

A film thickness monitor assembly 10 is illustrated in FIG. 1 inassociation with an inker 12 of a printing press 14. The film thicknessmonitor assembly 10 cooperates with the inker 12 to control thethickness of a liquid film of ink applied to a top plate cylinder 16 anda top blanket cylinder 18 of the printing press 14. Although the filmthickess monitor assembly 10 is used in the printing press 14 to controlthe thickness of a liquid film of ink, the film thickness monitorassembly could be used in other environments in association with eitherliquid or solid films.

In the printing press 14, information provided by the film thicknessmonitor assembly 10 in regard to the thickness of the film of ink beingapplied to the plate cylinder 16 is used to adjust the position of inkfountain keys 22. Adjusting the position of the ink fountain keys 22varies the amount of ink transmitted by a ductor roll 28 from an inkfountain roller 24 to a first roll 26 in a train of vibrator andintermediate rolls. The ink film transmitted from the fountain roll 24to the train of inker rolls is applied to the plate cylinder 16 by inkform rolls 32, 34, and 36.

Although only a single ink fountain key 22 has been illustrated in FIG.1, it should be understood that a plurality of ink fountain keys 22 arearranged in a linear array along an ink fountain blade. The ink fountainkeys 22 are actuated to control the amount of ink transmitted from areservoir 40 to the ink fountain roll 24 in a manner which is generallysimilar to that disclosed in U.S. Pat. No. 3,747,524. It should also beunderstood that although the film thickness monitor assembly 10 has beenshown in FIG. 1 as cooperating with the form roll 36, the film thicknessmonitor assembly 10 could cooperate with other rolls in the inker 12 tomonitor the thickness of the film of ink being applied to the platecylinder 16. For example, the film thickness monitor assembly 10 couldcooperate with the vibrator roll 42 if desired.

Film Thickness Monitor Assembly

The film thickness monitor assembly 10 includes a cylindrical sensorroll 46 (see FIGS. 1 and 2) and a cylindrical compliant roll 48 whichcooperate to define a nip 50 (FIGS. 3 and 4) through which a liquid film52 of ink passes (FIG. 4). The hollow sensor roll 46 has a cylindricalouter side surface 56 which engages one side of the ink film 52 (seeFIG. 4). The compliant roll 48 includes a plurality of annular members60 (FIG. 2) having cylindrical outer side surfaces 62 (FIG. 4) whichengage the opposite side of the ink film 52.

During operation of the printing press 14, the sensor roll 46 is rotatedabout its central axis by engagement with the rotating form roll 36(FIG. 1). The compliant roll 48 is rotated about its central axis, whichextends parallel to the central axis of the sensor roll 46 and form roll36, by engagement with the sensor roll. Although it is preferred to havethe drive forces for rotating the sensor roll 46 and compliant roll 48transmitted from the form roll 36 by friction drive between the rolls,it is contemplated that the sensor roll 46 and compliant roll 48 couldbe driven by gears which drive the rolls of the inker. It is alsocontemplated that the compliant roll 48 could be disposed in engagementwith the form roll 32.

Information indicative of the thickness of the ink film 52 istransmitted from the sensor roll 46 to control circuitry 64 (FIG. 1).The control circuitry 64 is connected with a plurality of motors 66 forrotating the ink fountain keys 22 to maintain a desired ink filmthickness.

The film thickness monitor assembly 10 includes a plurality of identicalmagnets 72 (see FIG. 2) which are arranged in a helical array 74 in thehollow sensor roll 46. The helical array 74 of magnets has only a single360 degree turn. Although only a few of the magnets 72 have been shownin FIG. 2, the helical array 74 of magnets includes one magnet for eachof the plurality of ink fountain keys 22 and rings 60 of the compliantroll 48. The helical array of magnets 74 extends around the sensor roll46 with each of the magnets 72 circumferentially offset relative to theadjacent magnets. The spatial relationship between the magnets 72results in a series multiplexing action in which only one of the magnets72 is adjacent to the nip 50, in the manner shown in FIGS. 3 and 4 atany given time.

Each of the magnets 72 includes a generally U-shaped core 76 (FIG. 4)formed of a material having a relatively high magnetic permeability. Inone specific embodiment of the invention, the core 72 was formed of astack of four laminations of a nickel-iron alloy. The laminations wereepoxyed together and had a thickness of approximately 0.006 inch. Theexterior of the U-shaped core was 0.7 inches across the back section 80and 0.6 inches along the leg sections 82 and 84. It should be understoodthat the foregoing composition and dimensions of the core 76 have beenset forth herein only for purposes of clarity of illustration and it iscontemplated that the core 76 could be constructed with a compositionand dimensions other than this specific composition and dimensions.

The core 76 of the magnet 72 is mounted on a cylindrical side wall 88 ofthe sensor roll 46 by forming openings 90 and 92 (FIG. 4) extendingthrough the side wall 88. The core 76 is positioned in the openings 90and 92 by bodies 94 and 96 with a suitable epoxy material. Although thecore 76 is formed of a material having a relatively high magneticpermeability, the wall 88 of the sensor roll 46 is formed of a materialhaving a relatively low magnetic permeability, such as aluminum.

An excitation coil 102 is wrapped around the back 80 of the core 76. Theexcitation coils 102 for the helical array 74 cf magnets areinterconnected in series and are connected with a constant source ofdirect current. The constant direct current source results in themagnetic excitation of each of the magnets 72 being constant.

Since the magnets 72 have a constant magnetomotive force, permanentmagnets could be substituted for the magnets 72 if desired. In onespecific embodiment of the invention, the excitation coil 102 wascomposed of 100 turns of number 33 magnet wire. The excitation coils 102for each of the magnets 72 were connected with a 10 volt DC source andwere connected in series with a resistance to approximate a constantcurrent source. There was a total resistance through the helical array74 of magnets of approximately 100 ohms and a constant magnitudeexcitation current of approximately 0.1 amperes. Of course, the specificconstruction of the excitation coil 102 and the magnitude of the directcurrent source and excitation current will be different for differentembodiments of the invention. A single coil may be used for bothexcitation and sensing.

Generation of Induced Voltage

A sensor coil 106 is provided in association with the magnet 72. Thesensor coil 106 responds to changes in the flux pattern of the magneticfield emanating from the magnet 72 as the magnet is moved toward andaway from an associated one of the rings 60 of material having arelatively high magnetic permeability. Thus, whenever the magnet 72moves toward the nip 50, the magnetic field emanating from the pole ends108 and 110 is concentrated by the associated ring 60 of material havinga high magnetic permeability. As the magnet 72 moves away from the nip50 in the space between the magnet and the associated ring 60 increases,the magnetic field disperses. The concentrating of the magnetic field asthe magnet 72 approaches the nip 50 and dispersing of the magnetic fieldas the magnet moves away from the nip 50 results in an induced voltagebeing generated in the sensor coil 106.

As the magnet 72 moves toward and away from the nip 50 during relativerotation between the rolls 46 and 48, the induced flux change in thesensor coil 106 varies in the manner indicated by the curve 114 in FIG.6. Thus, as the magnet 72 approaches the nip, the magnetic flux changein the sensor coil 106 increases in the manner indicated by the portion116 on the curve 114. When the magnet 72 is disposed in radial alignmentwith the annular ring 60, a maximum flux change 118 is generated in thesensor coil 106. As the magnet 72 moves away from the nip 50, theinduced flux change generated in the sensor coil 106 decreases in themanner indicated by the portion 120 of the curve 114.

The maximum flux change 118 generated in the sensor coil 106 will varyas an inverse function of the distance between the pole ends 108 and 110(FIG. 4) of the magnet 72 and the cylindrical outer side surface 62 ofthe annular ring 60, when the magnet 72 is at the nip 50. The distancebetween the pole ends 108 and 110 of the magnet 72 and the ring 60 whenthe magnet is at the nip 50, as shown in FIGS. 3 and 4, is equal to thethickness of the ink film 52. Therefore, the maximum flux changegenerated in the sensor coil 106 varies as an inverse function of thethickness of the ink film 52. Since the ink film 52 has a magneticpermeability which is very near unity, the only characteristic of theink film 52 which effects the magnitude of the flux change induced inthe sensor coil 106 is the thickness of the film.

The magnitude of the induced voltage is determined by the previously setforth equation: ##EQU6## Wherein: V is the voltage induced in thesensing coil.

M_(o) is the free space permeability.

N_(e) is the number of turns of the excitation coil 102.

N_(S) is the number of turns in the sensing coil 106.

I is the current in the excitation coil.

A is the area of the pole faces 108 and 110.

d is the gap or distance from the pole faces 108 and 110 to the surface62 of the ring 60.

The excitation current I is kept constant. A voltage proportional toflux change is produced if the voltage is integrated in a conventionaloperational amplifier circuit with input resistance R and feedbackcapacitor C, then the result is: ##EQU7## For any specific embodiment ofthe film thichness monitor assembly 10, only the integrated voltageV_(o) and the distance between the pole faces 108 and 110 and theassociated annular ring 60 will vary. Therefore, equation 6 can bewritten as follows: ##EQU8## Thus, when d is a minimum, that is when themagnet 72 is at the nip 50 as shown in FIGS. 3 and 4, the integral ofthe induced voltage in the sensor coil 106 is a maximum. By using solidfilms of a known thickness, the induced voltage generated in the sensorcoil 106 of a specific embodiment of the invention can be calibrated tocorrespond to a particular film thickness. However, one advantage ofthis method is that the value of K in equation 7 may be directlycomputed from the known parameters in equation 6 and calibration is notnecessary.

In the illustrated embodiment of the invention, the pole faces 108 and110 are exposed to one side of the ink film 52. This results in theminumum value of d in equations 6 and 7 being equal to the thickness ofthe ink film 52. However, it is contemplated that the magnets 72 couldbe mounted with the pole faces 108 and 110 inside the sensor roll 46. Ifthis was done, the minimum distance would be equal to the thickness ofthe ink film plus a constant equal to the distance which the pole faces108 and 110 are displaced from the side of the ink film.

Compliant Roll Construction

The steel rings 60 of the compliant roll 48 yield under the influence ofpressure applied against the rings by the film 52. Thus, each of therings 60 is mounted on a plurality of springs 124, 126, and 128 (seeFIG. 3). Therefore, each ring 60 is independently movable toward andaway from the sensor roll 46 with variations in ink film thickness. Thisenables each of the magnets 72 in the helical array of magnets 74 (FIG.2) to cooperate with an associated one of the rings 60 to generate aninduced voltage in a sensor coil 106 of a magnitude corresponding to theink film thickness in the portion of the nip 50 disposed between thering and the sensor roll 46.

A relatively thin ink film may be present at one location in the nip 50while a thicker ink film may be present at a location which is axiallyspaced from the one location. The fluid pressure forces applied againstthe rings 60 by the ink film 52 are sufficient to cause the ring 60adjacent to a relatively thick portion of the film to be radiallydisplaced from the nip 50. This allows the ink film monitor assembly 10to detect variations in the thickness of the ink film across the widthof the ink film.

A plurality of polymeric spacer rings 132 are disposed between the rings60 to hold the rings against axial movement and allow them to moveradially in and out relative to a cylindrical base roll 134. Although itis preferred to mount the sensing coils 106 on a core of each of themagnets 72, it is contemplated that the sensing coils could bepositioned at other locations. For example, the sensing coils could bepositioned in the annular rings 60 if desired. Although the annularrings 60 in the illustrated embodiment of the invention are formed of amagnetic steel, it is contemplated that the rings could be formed ofother materials having a relatively high magnetic permeability. Thus thecompliant roll could be formed of an elastomeric material containingnumerous particles of a material having a high magnetic permeability.

Control Circuitry

The electromagnets 72 are connected with a constant D.C. current source136 (FIG. 5) through slip rings 138 (FIG. 2). Therefore, the magnets 72have a constant magnetomotive force during rotation of the sensor roll46.

The control circuitry 64 (see FIG. 5) receives the output of the sensorcoils 106 for each of the magnets 72 in turn. Thus, the sensor coils 106are connected in series with each other. Since the magnets 72 aredisposed in a helical array in the sensor roll 46, the magnets 72 aremoved one at a time to the nip 50. Therefore, even though the sensorcoils 106 are connected in series, only one sensor coil 106 is providedan induced voltage output signal indicative of the thickness of aportion of the ink film 52 at any particular time during rotation of thesensor roll 46.

The sensor coils 106 are connected, through slip rings 140 (FIG. 2),with an operational amplifier 142 in a filter and integrating circuit144 through input resistors 146 and 148. An R-C feedback network 150 isconnected across the operational amplifier 142. The output from theoperational amplifier 142 is connected with a sampling circuit 154 and aphase synchronization circuit 156.

The output from the operational amplifier 142 has a waveform,illustrated at 160 in FIG. 5, with peaks corresponding to the maximuminduced voltage generated in each of the sensor coils 106 in turn.During operation of the printing press 14, the sensor roll 46 andcompliant roll 48 will be rotating at substantially constant speeds.Therefore, the voltage peaks of the waveform 106 will occur with auniform frequency.

The output from the synchronization circuit 156 is a series of voltagespikes, indicated at 162 in FIG. 5, of the same frequency as the voltagepeaks 160. The output from the synchronization circuit 156 causes thesampling circuit 154 to shift the level of a squarewave output,indicated at 164 in FIG. 5 to correspond to peaks of the induced voltageoutput signal from the operational amplifier 142. This results in themagnitude of the squarewave 164 varying in accordance with variations inthe maximum induced voltage generated in each of the sensor coils 106 inturn.

The output from the sampling circuit 154 is transmitted to ademultiplexer 166 through a function generator 168. The transferfunction for the function generator is d=K/v where v is the input to thefunction transfer generator 168. The output from the synchronizationcircuit 156 is transmitted to a counter 172. The counter 172 indexes thedemultiplexer 166 to transmit a voltage signal corresponding to thethickness of a film over each of a plurality of output lines in turn.

The voltage which is transmitted over each of the output lines 174 inturn effects actuation of the ink fountain keys 22 to maintain a desiredink film thickness between the sensor roll and each of the rings 60 onthe compliant roll 48. Thus, the voltage signal on the demultiplexeroutput line 174 designated as N in FIG. 5 is transmitted to a comparator176. The other input terminal of the comparator or operational amplifier176 is connected with a line 178 over which a voltage signalcorresponding to the desired thickness of the ink film is transmitted.If the signal on the line N representing the actual thickness of the inkfilm differs from the signal representing the desired thickness of thefilm, the ink key actuator 180 is energized to move the ink key 22 andadjust the ink fountain blade The ink key actuator 180 includes themotor 66 (see FIG. 1).

Although only the comparator 176 associated with the output linedesignated N has been illustrated in FIG. 5, similar comparators areassociated with the other output lines 174 and ink fountain keys 22.Therefore, each of the ink fountain keys 22 can be actuated to maintaina desired film thickness at each of the rings 60.

Second Embodiment

In the embodiment of the invention shown in FIGS. 1-4, the compliantroll 48 has rings 60 which are yieldably supported by springs 124, 126and 128. In the embodiment of the invention shown in FIG. 7, the ring ofmaterial having a high magnetic permeatibility is supported by a body ofelastomeric material. Since the embodiment of the invention shown inFIG. 7 is generally similar to the embodiment of the invention shown inFIGS. 1-4, similar numerals will be utilized to designate similarcomponents, the suffix letter "a" being associated with the numerals ofFIG. 7 to avoid confusion.

A compliant roll 48a includes a base roll 134a upon which a ring 60a ofmaterial having a high magnetic permeability is supported by a sleeve186 formed of elastomeric material. The sleeve 186 resiliently grips theouter side surface of the base roll 134a. An inner side surface 188 ofthe ring 60a is resiliently gripped by the outer side surface of thesleeve 186. The elastomeric sleeve 186 is radially yieldable to enablethe ring 60a to move radially relative to the base roll 134a. Aplurality of holes 190 are formed in the sleeve 186 to provide space forreceiving material of the sleeve as it is resiliently compressed byforces applied against the ring 60a by a film.

Summary

In view of the foregoing description it is apparent that the presentinvention provides a method and apparatus 10 for monitoring thethickness of a film. The apparatus 10 includes a sensor roll 46 and acompliant roll 48 which cooperate to define a nip 50 through which thefilm 52 passes. Each magnet 72 of a series 74 of magnets on the sensorroll 46 is moved in turn past the nip 50. Each of the magnets 72 is of aconstant magnetic strength and cooperates with a body of material havinga relatively high magnetic permeability the magnet passes by the nip 50.As a magnet 72 is moved by the nip, the magnetic field is concentratedand an induced voltage is generated in a sensor coil 106. The maximummagnitude of the induced voltage varies as an inverse function of thethickness of the film 52 at the portion of the nip 50 between the magnet72 and the compliant roll 48.

Although a film thickness monitor assembly 10 constructed in accordancewith the present invention can be used in many different environments todetect the thickness of either liquid or solid films, the assembly isadvantageously used in a printing press 14 to detect the thickness of aliquid film 52 of ink applied to a plate cylinder 16. When the filmthickness monitor assembly 10 is used in a printing press, the outputfrom the sensor coil 106 is transmitted to control circuitry 64 whicheffects operation of ink fountain keys 22 to maintain a desired filmthickness. The magnets 72 are advantageously arranged in a helical arrayin the sensor roll 46 to obtain a series multiplexing action due tomovement of each magnet in turn past the nip 50.

Having described specific preferred embodiments of the invention, thefollowing is claimed:
 1. An apparatus for monitoring the thickness of afilm, said apparatus comprising:a first member made from a materialhaving a relatively high magnetic permeability, said first member beinglocated on one side of the film; magnetic means for providing a magneticfield having a flux density and a substantially constant magnetomotiveforce, said magnetic means being located on the other side of the film;means, independent of the film, for effecting relative movement betweensaid magnetic means and said first member such that said magnetic meansand said first member periodically move to and away from a distancewhich is a function of the film thickness, said magnetic field of saidmagnetic means interacting with said first member such that the fluxdensity of said magnetic field varies in accordance with the spatialseparation between said magnetic means and said first member; and sensormeans for sensing variations in the flux density of said magnetic fieldduring said relative movement and for generating a signal indicativethereof, the generated signal when said first member and said magneticmeans are at said distance which is a function of the film thicknessbeing indicative of the film thickness.
 2. An apparatus as set forth inclaim 1 wherein said first member has surface means for engaging saidone side of the film, said apparatus further including a second memberhaving surface means for engaging the other side of the film at alocation opposite to a location where said surface means of said firstmember engages the one side of the film, said magnetic means beingcarried by said second member.
 3. An apparatus as set forth in claim 1wherein said first member has a first cylindrical side surface whichengages a first side of the film, said apparatus further including asecond member which is rotatable relative to the first member and has asecond cylindrical surface which engages a second side of the film, saidmagnet means being connected with and disposed within said secondmember, said means for effecting relative movement between said magnetmeans and said first member being operable to rotate said second memberto move said magnet means relative to said first member.
 4. An apparatusas set forth in claim 1 further including circuit means connected withsaid sensor means for providing control signals representative ofchanges in the thickness of the film and film thickness control meansconnected with said circuit means for varying the thickness of the filmas a function of the control signals to maintain a desired filmthickness.
 5. An apparatus as set forth in claim 1 wherein said magnetmeans includes a core member made from a material having a relativelyhigh magnetic permeability and excitation coil means connected with asource of direct current for magnetizing said core member to providesaid constant magnetomotive force.
 6. An apparatus as set forth in claim1 wherein said first member is a first roll which is rotatable about afirst axis and has first surface means for engaging a first side of thefilm, said apparatus further including a second roll which is rotatableabout a second axis parallel to said first axis and which has secondsurface means for engaging a second side of the film, said first andsecond surface means cooperating to define a nip through which the filmpasses, said magnet means including a plurality of magnet memberscarried by said second roll and disposed adjacent said second surfacemeans in a helical array about the second axis, said means for effectingrelative movement being operable to rotate said second roll about saidsecond axis to move each of said magnet members in turn to and from saiddistance which is a function of the film thickness.
 7. A method ofmonitoring the thickness of a film, said method comprising the stepsof:providing a first member made from a material having a relativelyhigh magnetic permeability and disposed on one side of the film;providing a magnet disposed on the other side of the film for providinga magnetic field having a flux density and a substantially constantmagnetomotive force, the magnetic field interacting with the firstmember such that the flux density of said magnetic field varies inaccordance with the spatial separation between said magnet and saidfirst member; effecting periodic relative movement between said magnetand said first member to and from a distance which is a function of thefilm thickness; and sensing the variations in flux density during suchperiodic relative movement and generating a signal indicatative thereof,the generated signal when the first member and said magnet are at saiddistance which is a function of the film thickness being indicative ofthe film thickness.
 8. The method of claim 7 wherein the step ofproviding a magnet includes the steps of:providing a core member madefrom a material having a relatively high magnetic permeability;surrounding said core member with an excitation coil; and energizingsaid excitation coil with a direct current thereby magnetizing said coreto provide said magnetic field having a flux density and a substantiallyconstant magnetomotive force.